The present invention relates to an image coder for arithmetically coding image data and, more specifically, to an image coder for arithmetic image coding which involves compression of binary image data for an image including text letters, diagrams, dot images, etc.
There is proposed a JBIG (Joint Bi-level Image Coding Experts Group) as an international standard coding scheme for compressing bi-level still image data which is applicable to image coders. The coding scheme makes standard, as an arithmetic code-based entropy code, the QM code which has been developed on the basis of Q code and Me 1 code.
A coder according to the JBIG usually uses a symbol occurrence probability table for the sake of simplicity of arrangement. In such a coder, the coding efficiency is enhanced, that is, the bit rate of codes becomes lower as an error between a predicted symbol occurrence probability obtained from the table and an actual occurrence probability becomes smaller. In light of this fact, Japanese Patent unexamined publication No. Hei.5-183,759 discloses a coder and a decoder having a plurality of such tables for selectively using one of them according to the kind of the image constituting each page of a document in coding and decoding.
Recently, however, it is not unusual for a document page to include dot images and/or diagrams as well as text letters. Further, a single page may include photographs which have been expressed in a pseudo gradation by means of dithering, error diffusing, etc. and are accordingly different in some characteristics such as statistical properties. Also, in a single image included in a document page, areas with homogeneous dot density will have different characteristics than areas where the dot density drastically changes at a contour. Thus, there is a problem in the prior art that areas of different characteristics in a page are coded and decoded with a same symbol occurrence probability table. Therefore, all the areas of a page are not necessarily coded or decoded optimally.